This invention relates to a slot antenna for a motor vehicle and particularly for a non-cavity-backed slot antenna in the roof of the vehicle suitable for commercial AM and FM radio reception. Such an antenna is linked with the vehicle body itself, and its characteristics are profoundly influenced by those of the vehicle body.
In the prior art, most vehicle mounted slot antennas have been disclosed in the vehicle trunk lid or as cavity backed antennas in the vehicle roof for directional signal locating purposes. The roof mounting for a slot antenna is superior to a trunk mounting because of the additional height of the antenna, which improves gain in both the AM and FM bands and which also removes it from the signal "shadow" of the upper portions of the vehicle body for an improved FM reception pattern. The lack of a cavity back for the antenna greatly reduces the capacitive loading of the antenna to enable reception at commercial AM frequencies, besides eliminating the bulk of the cavity from the vehicle roof.
There are several aspects of such a vehicle roof mounted slot antenna, however, which are critical to its performance but have not been shown in the prior art. A slot antenna of this type must be fed and grounded properly. There are several grounds to consider: DC ground, signal ground at AM frequencies and signal ground at FM frequencies. In addition, the optimum feed points may be different for signals in the commercial AM and FM broadcast bands. Finally, the material of the conducting members bordering the slots is also important in reducing the voltage standing wave ratio (VSWR) of the antenna.